Transfer-matrix simulations of electronic transport in single-wall and multi-wall carbon nanotubes

We present simulations of electronic transport in single-wall and multi-wall carbon nanotubes, which are placed between two metallic contacts. We consider situations where the electrons first encounter a singe-wall nanotube (corresponding to either the inner or the outer shell of the (10, 10)@(15, 15)@(20, 20) and (10, 10)@(20, 10)@(20, 20) nanotubes), before encountering the multi-wall structures. The role of this two-step procedure is to enforce the electrons to enter a single shell of the multi-wall nanotubes, and we study how from that point they get redistributed amongst the other tubes. Because of reflections at the metallic contacts, the conductance of finite armchair nanotubes is found to depend on the length of the tubes, with values that alternate between three separate functions. Regarding the transport in multi-wall nanotubes, it is found that the electrons keep essentially propagating in the shell in which they are initially injected, with transfers to the other tubes hardly exceeding one percent of the whole current. In the case where the three tubes are conducting, these transfers are already completed after four nanometers. The conductance and repartition of the current present then oscillations, which are traced to the band structure of the nanotube. The transfers between the shells and the amplitude of these oscillations are significantly reduced when the intermediate tube is semiconducting.